This invention relates to a battery pack containing a temperature sensor to detect battery temperature.
In a battery pack containing a temperature sensor, the temperature sensor detects battery temperature that has become abnormally high. This battery pack can be used safely because current flowing in the batteries is cut-off when battery temperature becomes abnormally high and exceeds a set temperature. Since a temperature sensor's electrical resistance changes with temperature, battery temperature can be detected by detecting electrical resistance. In the NTC (negative temperature coefficient) thermistor, which is one type of temperature sensor, electrical resistance decreases with increasing temperature. An NTC thermistor can be put in contact with a battery surface, and battery temperature can be detected by detecting the electrical resistance of the NTC thermistor. A battery pack containing a temperature sensor is described, for example, in the Japanese Patent Application Disclosure HEI 6-81425 (1994).
The battery pack disclosed in this patent reference houses a temperature sensor and a CPU (central processing unit). To reduce CPU power consumption, it is put in sleep-mode when no charging or discharging occurs over a one hour period, for example. Consequently, CPU power consumption is reduced when the battery pack is not used, and unnecessary battery discharge is prevented. However, for proper temperature detection, power consumption of the temperature sensor, which is a thermistor, is considerably large, and the system has the drawback that battery power is wastefully consumed. This is because thermistor electrical resistance is small at several tens of KΩ).
FIG. 1 shows a circuit using a thermistor to detect battery temperature. In the circuit shown in this figure, a series resistor 23 is connected to the thermistor 22 and voltage variation at the node connecting the thermistor 22 and series resistor 23 is detected to detect variation in the electrical resistance of the thermistor 22. In this circuit, when battery temperature becomes high and thermistor 22 electrical resistance changes, voltage at the node connecting the thermistor 22 and series resistor 23 increases. To accurately detect thermistor electrical resistance variation, namely battery temperature variation in this circuit, it is necessary to make the electrical resistance of the thermistor and the series resistor equal. However, if the thermistor 22 and series resistor 23 are equal, current flow in the series connection of thermistor and series resistor becomes large, and current consumed by the thermistor circuit becomes extremely large. Thermistor current can be reduced by increasing the size of the series resistor. However, if the electrical resistance of the series resistor is made very large with respect to thermistor resistance, it becomes impossible to accurately detect the electrical resistance variation of the thermistor. This is because voltage variation at the node connecting the thermistor and series resistor becomes small with respect to variation in thermistor electrical resistance. Consequently, for accurate detection of battery temperature, the electrical resistance of the series resistor cannot be made large, and power consumption by the thermistor circuit is high.
FIG. 2 shows a circuit wherein the thermistor circuit does not consume battery power. In the battery pack 30 of this figure, power is not supplied to the thermistor circuit 32 from the batteries 31. Power is supplied to the thermistor circuit 32 from a battery charger 33. In this circuit configuration, it is possible for the thermistor circuit 32 to not consume battery power, but it is only possible to detect battery temperature when the battery charger 33 is connected. Therefore, battery temperature can only be detected when the battery pack 30 is being charged. If battery temperature becomes abnormally high during charging of this battery pack 30, charging current can be cut-off. However, if battery temperature becomes abnormally high during battery discharge, current flowing through the batteries cannot be cut-off.